Treating steel in electric furnaces with basic hearths.



UNITED STATES PATENT OFFICE.

WILLIAM R. WALKER, OENEW YORK, N. Y.

No Drawing.

Specification of. Letters latcnt.

' Patented Feb. 10, 1914.

Application filed December 14, 1912. Serial No. 736,719.

To all whom it may concern:

Be it known that 1, WILLIAM R. WALKER, a citizen ofthe United States,residing in .the city, county, and State of New, York, have invented newand useful Improvements in Treating Steel in Electric Furnaces withBasic Hearths, of which the following is a specification.

In the making or refining of steel in electric furnaces, the usualmethod has been to make the lining or hearth of the furnace of basicmaterial, such as burnt magnesite or dolomite, theroof being of silicabrick. The slag used has been highly basic, containing-say about 15 percent. of silica, and requiring a considerable additio-nsay 30 per cent,of fluor. spar to act as'a flux. Electric furnaces with acid orsilicious hearths have been proposed, but up to this time have not beenput to any extensive practical use. In these acid electric furnaceoperations, the slag used has had a high silica content (52.75, 65.16and 60.46 per cent. in specific typical cases, as compared with a silicacontent of 15.95 per cent. in a specific basic slag for electricfurnaces). In the acid electric process, it is'necessary to use ore tooxidize the large excess of silicon reduced into the bath from thesilicious lining of the furnace and the highly silicious slag as used inthis process, a variable amount.

of oxid of 'iron being present in this silicidus slag and in the metalat the time of pouring the steel into the ladle. The temperatureattainable in the electric furnace is sufficiently high to melt a slagof extreme basicity, and the fumes from the highly basic slagsgenerallyused have a strong corrosive action on the silica of the roof, so thatthe latter requires renewing at frequent intervals, resulting inconsiderable expense and very serious delays in the op eration of theelectric furnace. The fluor spar which becomes volatilized has the samecorrosive effect on the roof, and this material furthermore is quiteexpensive. The chief reason for using a slag so highly basic, has beento reduce to a minimum the sulfur contentof the product. I have found,however, that by using a slag containing a comparatively high percentageof silica, the sulfur content of the metal may be reduced to practicallythe same extent as before and considerable economies are effected in theoperating of the furnace and also an improvement secured in the qualityof the steel produced. In these revious basic processes, the percentageof sil1ca-has been so low that if any of it was reduced to silicon, suchre duction wasonly temporary, the oxids present being sufficient toreconvert it into silica, so that it remained in or returned to the slagand there was no appreciable increase of silicon in the producttraceable to the silica in the slag. I propose to increase the quantityof silica in the sla to'such an extent that a part thereof will bereduced and remain in the metal, at the same time keeping the percentageof silica so low that it will not cut the hearth to any substantialextent.

The operation is a reducing one at all times. The silicon enters thebath while wholly, or largely, in the nascent condition and, therefore,in a condition most active in eliminating oxide and other deleterioussubstances which may be in the bath. There is no addition of ore (as inthe acid rocess) to remove surplus silicon and i must, therefore, avoidintroducing a surplus of silica and must keep within the limit at whichthe slag would seriously scorify the hearth. I cannot, therefore, alwayssecure all the silicon that I desire in the product, from the silicaintroduced into the slag, but I can secure a substantial percentage ofsilicon in the product in this way, thus effecting a considerableeconomy. The actual percentage of silica in the slag will varyconsiderably for different materials and circumstances, but willgenerall be between 25 and 35 per cent. of'the tota weight of the slag.The percentages cannot be stated with absolute accuracy; and the slagbesides comprising lime and silica may also contain other elements, suchfor example as alumina. Due consideration must be given to the fact thatthe commercial lime and silica are not pure and that other impuritieswill be introduced to a greater 'or less extent from the metal itselfand from the hearth. The mixture of lime and silica should beapproximately neutral (that is neither strongly basic nor stronglyacid); then as silica is reduced to silicon there is resent in the slaga percentage of lime su cient for desulfurizing. A good practicalexample is a slag made of silica 30 per cent. and lime (approximatelypure CaO) to whichi should be added coke (or other carbonaceousmaterial) at least 5 per cent. by weight of the combined silica andlime. This slag will also contain magnesia and alumina, which arederived from the furnace bottom materials and the lime additions.

Besides the advantages above referred to, the invention has certainother important advantages. In the first place, the slag with theincreased percentage of silica is more fusible. Furthermore, thequantity of fluor spar necessary may be reduced to little or none, thussaving a considerable expense. Also, by suitably proportioning thesilica to the lime, 1 can minimize the formation of fumes in the furnaceso that when the ingredients of the slag are in the-best proportions,one can look into the furnace without seeing any of the fumes whichgenerally fill the space above the metal. Working with the 30 per cent.silica slag above described, I have found that the roof of the furnacewill last much longer than with the previously used basic slags undersimilar conditions. Where the sides ofthe furnace are of silica brick,as is sometimes the case, approximately the same increased life isobtained as for the silica roof.

The method may be used for refining or. working metal brought molten tothe electric furnace or it may be used in processes in which the metalis put into the electric furfiace cold and melted down therein, or onrefined or partially refined metal coming from a basic Bessemerconverter or basic open hearth furnace, or the refined or partiallyrefined metal from the acid converter or acid open hearth furnace; inthe two latter cases, first using an oxidizing slag of ordinarycomposition for the removal of the remaining carbon, silicon andphosphorus and then following with the finishing slag described in thepresent invention. This refining slag is melted preferably by electricheat and can be more easily melted than the slags that have beenpreviously used. The previous slag has required about thirty minutes formelting and the new slag requires only about fifteen minutes. Thissaving of time means a very substantial economy in the cost of the steelproduced. The slag is a finishing slag and, by reason of its greatliquidity and the consequent freedom with which it circulates, has amore thorough purifying effect on the metal. The slag being more liquid,small particles which are inclgmed in the metal rise to the surface morereadily, thus leaving the bath in an extremely pure metallic condition.The more liquid slag flows more readily from the furnace after the metalhas been removed.

The invention is designed especially for use in connection with electricarc furnaces, of the Heroult arc type for example, because in thesefurnaces it is easy to secure the high temperature desired and toobserve and reg ulate the working of the process.

The process is of particular advantage in the manufacture of soft or lowcarbon steel. The greater or less quantity of nascent silicon whichenters the bath serves to purify it and in a way to make up inpurifyingeffect for the lack of carbon in the bath. The silica in the slag may beso proportioned that the quantity of nascent silicon which enters thebath shall be only sufficient to have a purifying effect and shall notbe sufficient to leave any appreciable percentage of silicon in thefinished product.

Collectively, the several advantages above specified mean a veryimportant economy in the'production of electricallyrefined steel. Theinvention also enables me to produce a steel that will be more uniformlyfree from slag inclosures and also more thoroughly deoxidized and freefrom deleterious substances, than is possible with previous processes.The deoxidation and freedom from such deleterious substances is the mostimportant element. in the production of steel of the highest quality; Bythis process it is possible to produce steel of a quality similar tothat produced in the so called crucible process at a cost that will notbe prohibitive even for heavy products, such for example as rails,axles, ties, etc., and also for soft steel, such as boiler plate andbridge and structural shapes.

What I claim is:

1. The method of treating iron or steel which consists in applyingthereto a slag comprising lime and silica and maintaining the metal andslag molten on a basic hearth in an electric furnace, the quantity ofsilica being about 25 to 35 per cent. of the total weight of the slag.

2. The method of treating iron or steel which consists in applyingthereto a slag comprising lime and silica and maintaining the metal andslag molten on a basic hearth in an electric furnace, the lime andsilica being approximately in such proportions as to form a neutralmixture, that is, a mixture which is neither strongly acid nor stronglybasic.

3; The method of treating iron or steel which consists in applyingthereto a slag comprising lime and silica and maintaining the metal andvslag molten on a basic hearth in an electric furnace, the percentage ofsilica in the slag being so low as to avoid serious scorifying of thehearth and being so high that silicon is reduced therefrom and isincorporated in the product.

4. The method of treating iron or steel which consists in applyingthereto a slag comprising lime and silica and maintaining the metal andslag molten on a basic hearth in an electric furnace, the silica andlime cient lime to practically eliminate sulfur from the product.

5. The method of treating iron or steel which consists in applyingthereto a slag comprising lime and silica and maintaining the metal andslag molten on a basic hearth in an electric furnace, the percentage ofsilica in the slag being sufficient to practically avoid formation offumes of lime in the furnace.

6. The method of treating iron or steel which consistsin applyingthereto a slag comprising lime and silica and maintaining the metal andslag molten on a basic hearth in an electric furnace, the slagcomprising about 30 per cent. silica and 70 per cent. lime, to which areadded about 57per cent. by Weight of carbonaceous material.

7. The method of'treating iron or steel which consists in" melting thesame on a basic hearth in an electric furnace with an oxidizing slag andthus taking out carbon, silicon and phosphorus, removing said slag andapplying to the bath a finishing slag comprising lime and silica inapproximately such proportions as to form a neutral mixture, that is amixture Which is neither strongly acid nor strongly-basic, and finishingthe metal with such slag.

8. In the making of soft or low carbon steel the methodwhich consists inapplying a to the bath a slag comprising lime and silica and maintainingthe metal and slag molten on a basic hearth in an electric-furnace,the-- percentage of silica in the .slag being so high that silicon isreduced therefrom and enters the bath in nascent condition.

-9. In the making of soft or low. carbon steel the method which cpnsistsin applying to the bath a slag comprising lime and silica andmaintaining the metal and slag molten on a basic hearth in an electricfurnace, the

percentageof silica in the slag being so high that silicon is reducedtherefrom and enters the bath in nascent condition in sufficientquantity to remove impurities to a substantial extent but not insufiicient quantity to appear in an appreciable percentage in thefinished product.

In witness whereof, I have hereunto signed my name in the presence oftwo subscribing witnesses.

, WILLIAM R. WALKER,

Witnesses D. ANTHONY USINA, LULU STUBENBOLL.

